Replace license blurbs with SPDX tags

[zyv]

Is there an existing request for this?

• I have searched the existing issues

Is your feature request related to a problem?

As discussed in #4709, we have the following issues:

1. Huge license blurbs in every file
2. Automated processing is not possible
3. Years need to be maintained

Describe the solution you'd like

Use SPDX instead:

/* SPDX-License-Identifier: GPL-3.0-or-later */
/* Copyright Free Software Foundation, Inc. and contributors to the GNU Midnight Commander project. */

Describe alternatives you've considered

No response

Additional context

No response

[ossilator]

spdx use is orthogonal to keeping the copyright year in the files.
i have no opinion on whether the update ritual has any practical meaning.

the copyright statements can also be formalized.

practical example:

// SPDX-FileCopyrightText: 2000-2002 Michael R. Elkins <me@mutt.org>
// SPDX-FileCopyrightText: 2002-2022 Oswald Buddenhagen <ossi@users.sf.net>
// SPDX-License-Identifier: GPL-2.0-or-later WITH LicenseRef-isync-GPL-exception